Varactors for High-Sensitivity Reflectometry of Quantum Dots*

The highest-sensitivity readout of spin qubits is typically achieved using radio-frequency (RF) reflectometry techniques. Such methods include RF-single electron transistors (RF-SETs) and gate-dispersive charge sensing. The characteristic impedance of standard RF equipment is 50 W, whereas typical quantum devices impedances are on the order of the quantum resistance h/e² » 25.8 kW or higher in the case of gate-sensors. Off-chip tank circuits are used to overcome this mismatch but lack in-situ tunability to compensate for temperature-dependencies and device-to device variability. Earlier work [1] has shown that commercial GaAs varactor diodes can be used to in-situ tune the tank circuit to perfect matching, improving sensitivity and measurement speed. However, such voltage-tunable capacitors freeze out at temperatures below 1K and show a significant dependence on magnetic fields. Using quantum paraelectric materials like Strontium Titanate (STO) alleviates these problems [2], but provides only limited tunability compared to GaAs. Here, we report on the characterization of Strontium Titanate (STO) ring-varactors which robustly reach hyperabrupt capacitance tunability. We use those devices to gate-dispersively measure hole quantum dots in GeSi core-shell nanowires and fin-field effect transistors, both promising platforms for spin qubits [3,4].

[1] Ares et al. PRA 5, 034011 (2016)

[2] Apostolidis at al. arXiv:2007.03588

[3] Froning et al. Nat Nano. 16, 308-312 (2021)

[4] Camenzind et al. Nat Electron. 5.3 178-183 (2022)